1. Field of the invention
The present invention relates to an optical waveguide coupler device presenting the general configuration of a slightly unbalanced MachZehnder interferometer. The present invention also extends to a method for forming such a coupler device on laterally adjacent optical waveguides.
2. Brief description of the Prior Art
Tapered 2.times.2 couplers formed by fusing together sections of two laterally adjacent optical fibers and by subsequently stretching these fused sections, are well known and versatile devices that can be used in many applications where given coupling ratios at a given wavelength of the propagated light signal is required.
These couplers generally present good performances; they have small dimensions, cause low losses, present good thermal stability, and can be easily and rapidly produced.
The prior art all-fiber fused couplers used as power splitters are sensitive to optical wavelength. Indeed, their coupling ratios are dependent on the wavelength which is itself directly dependent on the degree of stretching and the outline of the coupler.
Wavelength dependent couplers are useful in wavelength multiplex communication systems which simultaneously transmit a plurality of messages on a single fiber. Indeed, these communication systems require couplers capable of separating light signals of different wavelengths (for example the currently used 1300 and 1550 nm wavelengths). However, wavelength independent couplers are required for example when the light source used in a communication system has a relatively wide spectral bandwidth.
To flatten the wavelength response of such a coupler, it is known to fuse and stretch two optical fibers of different diameters or two identical fibers made dissymetric by pretapering or etching of one of them. Production of these couplers is however complicated by the dissymetry of the fibers, and by the accurate control required by the pretapering or etching.